Effect Of Molecular Architecture Research Articles

Effect of Molecular Architecture on Dynamics of Multigraft Copolymers: Combs, Centipedes, and Barbwires

An investigation was carried out on the effect of molecular architecture on the dynamics of multigraft (MG) copolymers of polyisoprene (PI) and polystyrene (PS). MG copolymers with regularly spaced...

Diblock Copolymer Reinforced Interfaces between Amorphous Polystyrene and Semicrystalline Polyethylene

The effects of molecular architecture on the fracture properties of semicrystalline polymers were probed at diblock copolymer-reinforced interfaces between polystyrene (PS) and polyethylene (PE). The PE used for this study was a model ethylene−butene copolymer which was chosen for its compatibility with hydrogenated 1,4-polybutadiene. This compatibility allowed the use of hydrogenated poly(styrene-b-1,4-tetradeuteriobutadiene) as the block copolymer. For a series of these diblock copolymers, the areal chain density (Σ) and the molecular weight of the PE block (Mn) were varied systematically to observe their effects on the interfacial fracture energy (Gc). At low Σ, Gc stayed relatively constant, and was roughly 1 J/m2. Above a critical value of Σ, the fracture energy climbed rapidly. This critical value decreased with increasing Mn. The detection of deuterium on the fracture surfaces indicated that pullout of the PE block was the predominant failure mechanism when Mn ≤ 30 kg/mol. Only when the molecular w...

Physical gelation of crystallizing metallocene and Ziegler-Natta ethylene–hexene copolymers

The effect of molecular architecture on the evolution of viscoelastic properties during crystallization was investigated using ethylene–hexene copolymers manufactured via metallocene (M-LLDPE) and Ziegler-Natta (ZN-LLDPE) processes. Differences in branching distribution were shown to have a drastic effect on the viscoelastic properties near the gel point. It is shown that the branching distribution rather than branch content is the determining parameter for the evolution of the rheological properties during isothermal and non-isothermal crystallization, and for the width of the solidification interval. We developed a partial melting technique for the preparation of stable critical gels of LLDPE whose viscoelastic properties correspond to the intermediate state between melt and solid. Local molecular conformation and crystallinity in these gels were characterized by Raman spectroscopy, which shows that the transition from melt-like to solid-like rheological behavior (physical gelation) in LLDPE occurs at a very low overall crystallinity of less than 5%.

Roles of Molecular Architecture and End-Group Functionality on the Surface Properties of Branched Polymers

The effects of molecular architecture on the surface properties of polyetherimide polymer films were investigated. For polymers ranging in structure from hyperbranched to completely linear, it was ...

Structure modification and crosslinking of methacrylated polylactide oligomers

AbstractThe effect of molecular architecture on the properties of crosslinked polylactides was studied. D,L‐Lactide‐based telechelic oligomers with different numbers of arms were functionalized with methacrylic anhydride. Functionalized oligomers were crosslinked with thermal initiation at 90°C, and the mechanical and thermal properties, as well as the gel content, were evaluated. The crosslinking density increased with the number of arms in the oligomers, and the compressive yield strength was built up at the same time. The best mechanical properties (compressive yield strength = 120 MPa, modulus = 2800 MPa, and strain = 5.0%) were obtained with cured polymers prepared with a 100 : 8 ratio of lactide to pentaerythritol. When dimethacrylated butanediol was used as a reactive monomer in curing, polymers with high crosslinking density and mechanical strength were obtained at 37, 60, and 90°C. The use of the reactive monomer also enabled high conversions with photoinitiated crosslinking. Finally, hydrolytic degradation and strength retention were demonstrated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3616–3624, 2002

Synthesis of perylene-porphyrin dyads for light-harvesting studies

The spectral coverage of porphyrin-based light-harvesting arrays can be enhanced through the use of suitable accessory pigments. Perylene-monoimide dyes can serve as valuable accessory pigments with porphyrins. To investigate the choice of perylene-monoimide and the effects of molecular architecture on light-harvesting efficacy, five perylene-porphyrin dyads were prepared. Each dyad employs a diphenylethyne linker that bridges the perylene N-imide site and the porphyrin meso-position. Three dyads incorporate a mono-phenoxy perylene at the o-, m-, or p-position of the meso-aryl group on the porphyrin. The two remaining dyads incorporate a perylene-monoimide (bearing zero or three phenoxy substituents) at the p-position of the meso-aryl group on the porphyrin. The introduction of phenoxy groups on the perylenes increases the solubility, a key requirement for use in light-harvesting arrays. The long-wavelength absorption band of the perylene shifts from 506 nm to 532 or 533 nm upon substitution with one or three phenoxy groups, respectively. The synthesis of the dyads entails Pd -mediated coupling of a bromo-perylene and an ethynyl porphyrin, or the mixed-aldehyde condensation with a perylene-aldehyde, mesitaldehyde, and pyrrole. Five perylene-monoimide dyes bearing an ethyne or bromo substituent at the p-position of the N-aryl unit were developed for this modular chemistry. Each perylene-porphyrin dyad exhibits efficient energy transfer from the excited perylene to the ground-state porphyrin.

Effects of molecular architecture on two‐step, melt‐spun poly(lactic acid) fibers

AbstractFibers of poly(lactic acid) (PLA) produced by two‐step melt spinning have been studied. The PLA resins used contain a 96:04 ratio of L:D stereochemical centers; however, one of the materials is branched by a peroxide treatment. The thermal, mechanical, and morphological properties of the fibers are compared for the two different molecular architectures. In the branched material, at least some of the branches exceed the entanglement molecular weight. The branched material is accordingly characterized by greater shear and extensional viscosity than the linear material. Fiber properties are highly influenced by the draw ratio; both branched and linear materials reach a plateau of about 35% crystallinity. The branched polymer reaches the plateau at a lower draw ratio, and this is indicative of faster crystallization kinetics. Both materials shrink in boiling water, and the amount of shrinkage decreases with increasing draw ratio. At an intermediate draw ratio of 6, the branched material is characterized by significantly larger shrinkage. With small‐angle X‐ray scattering and atomic force microscopy, the morphology is found to be fibrillar. Microfibril diameters range from approximately 20 to 30 nm and are almost identical for the two molecular architectures studied. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2839–2846, 2002

A parallel algorithm for molecular dynamics simulation of branched molecules

To get an insight into the effects of molecular architecture in the behaviour of thin lubricant films we have devised an algorithm for simulation of branched molecules. We have used this algorithm successfully to simulate branched isomers of C 30. However the algorithm is flexible enough to be used for the simulation of more complex branched molecules. The resulting algorithm can be used in molecular dynamics simulation of branched molecules and could be helpful in designing new materials at the molecular level.

Chemically tuning the optoelectronic properties of terphenylene-containing block copolymers

A series of partially conjugated polymers containing terphenylene linked by vinylene units were synthesized by Wittig condensation polymerization of the appropriate diphosphonium salts and the dialdehyde monomer. meta-Phenylene, para-phenylene, 1,3,4-oxadiazole-2,5-diyl-1,4-phenylene, 2,5-dimethoxy-1,4-phenylene and 9,10-anthrylene units were incorporated into the vinylene blocks to control the band gaps. The effect of molecular architecture on optoelectronic and thermal properties of the polymers was investigated. The emission of the copolymers can be tuned by changing the nature of the vinylene blocks to show violet, blue, green and green-yellow. Double-layer LEDs with ITO/PEDOT/polymer/Ca/Al structures were fabricated and, in parallel with the photoluminescence results, the change of emission color was also shown in the electroluminescence spectra. Electroluminescence (EL) spectra of ITO/PEDOT/polymer/Ca/Al devices.

Molecular mobility of elastin: effect of molecular architecture.

The thermal and dielectric properties of elastin and two soluble derivatives (kappa-elastin and derived elastin peptides from enzymatic elastolysis) were investigated in the freeze-dried state in a wide temperature range (from -180 to +220 degrees C). The glass transition of these amorphous proteins was studied by differential scanning calorimetry (DSC). The dielectric relaxations of both proteins were followed by thermally stimulated currents (TSC), an isochronal dielectric spectrometry running at variable temperature, analogous to a low-frequency spectroscopy (10(-3)-10(-2) Hz) and by dynamic dielectric spectroscopy (DDS), performed isothermally with the frequency varying from 10(-2) to 3 x 10(6) Hz. The combination of TSC and DDS experiments and the determination of the activation parameters of the relaxation times inform about the molecular mobility of the proteins, both in the glassy state and in the liquid state. Major differences between the relaxation behavior of elastin and its soluble derivatives have been discussed and correlated with the molecular architecture of the proteins.

The effect of branching on slip and rheological properties of lubricants in molecular dynamics simulation of Couette shear flow

Molecularly thin liquid films of alkane in extreme conditions in a thin film lubrication regime have been investigated. To get an insight into the effects of molecular architecture in the behaviour of these thin lubricant films we have studied six different molecules, mainly isomers of C 30. In this work the effect of branching on rheological properties and behaviour of lubricant film is examined. Our study shows viscosity and normal stress effects depend on the degree of branching. Dynamics of the molecules and their orientation are also affected by the degree of branching. A weaker layering near the wall is observed for branched molecules. Slip between the wall and lubricant film also was larger for branched molecules. Branched molecules had less tendency to change their orientation under the flow. The results obtained here could be helpful in designing new lubricants at the molecular level.

Isothermal Curing of Acetylene Functionalized Liquid Crystalline Thermoset Monomers

To investigate the effects of molecular architecture on the liquid crystalline phases and times to gelation during isothermal curing, two homologous series of acetylene functionalized thermotropic liquid crystalline monomers were synthesized and examined. New liquid crystalline phase−time−temperature−physical transformation diagrams (LCPTTT) were constructed for four of the monomers and illustrate that the monomers change from nematic liquids to isotropic or biphasic gels during isothermal curing. However, reemergence of an ordered phase occurred if the monomers were cured at low enough temperature. The critical temperature for ordered phase retention in the final vitrified material is inversely proportional to the length of the terminal flexible chain. The time to gelation for a single set of isothermal cure conditions did show an increase in gel times with increasing chain length; however, an odd−even effect was seen in both series rather than a linear increase.

Physical and mechanical properties of polyethylene for pipes in relation to molecular architecture. I. Microstructure and crystallisation kinetics

Ethylene/α-olefin copolymers having bimodal molar weight distribution are investigated in comparison with unimodal copolymers in order to understand the incidence of the molecular architecture on the stress cracking resistance. The preferred introduction of the co-units in the longest chains of bimodal copolymers is suggested to favour the occurrence of intercrystalline tie molecules during crystallisation. The more complex is the molecular architecture, the greater is the difficulty for crystallisation by regular chain folding. Intermolecular chemical heterogeneity resulting from preferred incorporation of the co-units in the long chains enhances the co-unit disturbing effect on crystallisation without reducing crystallinity. Intra-molecular heterogeneity of the co-unit distribution is also suggested to be an efficient means to generate tie molecules and random chain folding at the expense of regular chain folding. Isothermal crystallisation is used to probe the effect of molecular architecture on the crystallisation kinetics. It appeared that the correlations between kinetics, molecular architecture and molecular topology of unimodal copolymers no longer hold when considering bimodal copolymers. In contrast, the crystal surface free energy proved to be sensitive to topological changes resulting from molecular architecture modifications.

Rheology of metallocene‐catalyzed monomodal and bimodal polyethylenes

AbstractThe effect of molecular architecture on the dynamic viscoelastic properties of new metallocene high density polyethylenes has been analyzed. Bimodal molecular weight distribution metallocene polyethylenes show features different from conventional polydisperse and bimodal polyethylenes. Higher values of Newtonian viscosity (ηo) at the same values of weight average molecular weight (Mw) and stronger frequency dependence of dynamic viscosity (η′) than in conventional HDPE‐s have been observed; this leads to lower values of the characteristic frequency for the onset of non‐Newtonian behavior (ωo) and higher values of the power law index (α). These features are probably due to the presence of very small amounts of long chain branching (LCB). The implications of these results in polymer processing are analyzed comparing extrusion rheometer data, which leads to the conclusion that extrusion difficulties in metallocene catalyzed polyethylenes can be overcome with bimodal molecular weight distributions.

Interfacial behaviour of 'new' poly(ethylene oxide)-containing copolymers

Block copolymers containing poly(ethylene oxide) (PEO) have a wide applicability within biomedical applications, not the least due to anti-fouling properties of surface coatings based on these copolymers. We have investigated a number of these, and results for PEO/poly(butylene oxide) (PEO/PBO), PEO/poly(lactide) (PEO/PL), and PEO/poly(ethylene imine) (PEO/PEI) copolymers, as well as for PEO-esterified fatty acids, are presented and discussed. For the former class of polymers, the effects of molecular architecture on the adsorption properties are adressed, and experimental results obtained with ellipsometry and small-angle neutron scattering are presented. For the PEO/PL block copolymers, the effects of the PEO and PL lengths for the polymer adsorption are addressed, as are the effects of degradation of the PL moiety on both adsorption and protein rejection. For the PEO-esterified fatty acids, the effects of PEO chain length and interfacial density on the protein rejection capacity of such coatings are discussed.

π-Shaped double-graft copolymers: effect of molecular architecture on morphology

The morphologies formed by block copolymers with a double-graft, π or (S,I)I′(S,I) architecture were investigated using transmission electron microscopy (TEM) and small angle neutron scattering (SANS). Here S and I represent blocks of polystyrene and polyisoprene, respectively. These materials were synthesized using anionic polymerization and chlorosilane linking, and they were characterized using size exclusion chromatography, membrane osmometry, and low-angle laser light scattering. This characterization work confirmed the desired molecular architectures and narrow molecular weight distributions. The results of morphological characterization indicate that one can understand complex grafting architectures by decomposing them into fundamental building blocks, which are taken as the component single-graft structures out of which the larger structure is constructed. We propose rules for dividing structures into these components, which we call constituting block copolymers. For the π double-graft architecture, the constituting block copolymer is an asymmetric single-graft block copolymer. The morphological behaviour of the more complex double-graft architecture is approximately equivalent to that of the constituting single-graft block copolymer. Through the use of the constituting block copolymers we map the experimentally determined morphological behaviour of eight materials with π architecture onto the morphology diagram calculated by Milner for miktoarm stars. As in our previous study of asymmetric single-graft block copolymers, the asymmetry of the graft location along the backbone requires a generalization of the molecular asymmetry parameter, ε, used in the calculated morphology diagram.

Effect of molecular architecture of hydrophobically modified poly( N-isopropylacrylamide) on the formation of thermoresponsive core-shell micellar drug carriers

Terminal-incorporation of hydrophilic or hydrophobic groups dramatically influences the phase transition of poly( N-isopropylacrylamide) (PIPAAm) because of a critical role of the polymer chain ends in initiation of the phase transition. Incorporation of an amino or hydroxyl group to one end of PIPAAm remarkably raised the LCST (lower critical solution temperature) and slowed down the rate of the phase transition, and these effects were more pronounced as the mole fraction of hydrophilic groups increased compared to the random copolymers of PIPAAm and hydrophilic co-monomers, such as acrylic acid (AAc) or dimethylacrylamide (DMAAm). Hydrophilic effects were more remarkable for hydroxyl groups, due to stronger hydrogen bonding with water. Terminal-modification (hydrophobization) was also more effective in producing hydrophobic effects on the PIPAAm phase transition in comparison with PIPAAm copolymers that were randomly modified along the main chain with hydrophobic co-monomers. Moreover, terminal-located hydrophobic groups were able to form hydrophobic microdomains that were clearly isolated from PIPAAm chains in aqueous media by the aggregation of hydrophobic segments. As a result, the obtained micellar aqueous solution showed the same LCST as pure PIPAAm, while the PIPAAm random copolymer with hydrophobic co-monomers formed incompletely separated microdomains.The LCST for this random copolymer was reduced with increasing hydrophobic co-monomer mole fraction. Hydrophobically terminal-modified PIPAAm produced thermo-responsive core-shell structures that exhibited the same LCST and the same thermal response rate as those of free linear PIPAAm chains. Such polymeric micellar structures show reversible thermoresponsive aggregation/dispersion and deformation/reformation in heating/cooling cycles through the LCST for pure PIPAAm. These properties indicate the possibility of using such a system as a thermoresponsive drug carrier with double targeting mechanisms, in both passive and active manners.

The Effect of Molecular Architecture on the Thermotropic Behavior of Poly[11-(4‘-cyanophenyl-4‘‘-phenoxy)undecyl acrylate] and Its Relation to Polydispersity

Contrary to theoretical predictions on rodlike molecules (mixture of axial ratios) and previous experimental speculations on side chain liquid crystalline polymers (mixture of molecular weights), the breadth of the isotropization transition of poly[11-(4‘-cyanophenyl-4‘‘-phenoxy)undecyl acrylate] is not broadened by polydispersity in chain length alone. Instead, it is broadened by the limited miscibility of a mixture of branched structures. SCLCPs and their mesogenic side chains can therefore be treated as mixtures of random coils and monodisperse rods. This was demonstrated by comparing the thermotropic behavior of linear and three-arm star poly[11-(4‘-cyanophenyl-4‘‘-phenoxy)undecyl acrylate]s prepared by atom-transfer radical polymerization (ATRP), and their binary blends and unmixed composites. The biphasic region of linear polymers with pdi = 1.15−1.49, and of their binary mixtures, is extremely narrow. Although 3-arm star polymers with pdi = 1.11−2.20 also exhibit extremely narrow isotropization transitions, binary mixtures of the star polymers with significant differences in branching density have limited miscibility and broad isotropization transitions. The broad isotropization transition of unfractionated poly[11-(4‘-cyanophenyl-4‘‘-phenoxy)undecyl acrylate] prepared by standard radical polymerization indicates that it also contains a mixture of branched structures due to chain transfer to polymer at high monomer conversion. 11-(4‘-Cyanophenyl-4‘‘-phenoxy)undecyl acrylate is the most highly functionalized monomer to be polymerized by ATRP, and the resulting 3-arm star polymers are the first star polymers synthesized by ATRP.

Effect of molecular architecture on the electrorheological behavior of liquid crystal polymers in nematic solvents

Effect of molecular architecture on the electrorheological behavior of liquid crystal polymers in nematic solvents

H-shaped double graft copolymers: Effect of molecular architecture on morphology

The morphologies formed by block copolymers with a double-graft, H or S2IS2 architecture were investigated using transmission electron microscopy (TEM) and small angle neutron scattering (SANS). Here S and I represent blocks of polystyrene and polyisoprene, respectively. These materials were synthesized using anionic polymerization and chlorosilane linking, and they were characterized using size exclusion chromatography, membrane osmometry, and low-angle laser light scattering. This characterization work confirmed the desired molecular architectures and narrow molecular weight distributions. The results of morphological characterization indicate that one can understand complex grafting architectures by decomposing them into fundamental building blocks, which are taken as the component single graft structures out of which the larger structure is constructed. We propose rules for dividing structures into these components, which we call constituting block copolymers. The morphological behavior of the more complex architecture is approximately equivalent to that of the constituting block copolymer structure. Through the use of the constituting block copolymers we map the experimentally determined morphological behavior of the H architecture onto the morphology diagram calculated by Milner for miktoarm stars [Macromolecules 27, 2333 (1994)]. Mapping the H architecture onto the morphology diagram in this way produces general agreement between experimental results and the model. However, it is found that in the case of the H architecture, as well as in previously published results for I2S and I3S miktoarm star materials, that the morphology diagram slightly overestimates the amount of shift in the order-order transition lines produced by asymmetry in molecular architecture. This overestimation in the theory is attribute to a junction point localization effect which was neglected in Milner’s calculation.